Continuous-current transformer



July 5, 1927. 1,635,002

- T. C LENNOX CONTINUOUS CURRENT TRANSFORMER Filed Aug. 20, 1925 3Sheetg-Sheetl Tnventor': v Thomas C.L enno H L5 Attorney 1,635,002 y 5,2 T. c. LENNOX CONTINUOUS CURRENT TRANSFORMER Filed Aug. 20, 1925 3Sheets-Sheet 2 Tnventm: Thomas QLennOX,

HLs Attorneg.

July 5,1927. 1,635,002

T. C. LENNOX CONTINUOUS CURRENT TRANSFORMER Filed Aug. 20, 1925 sSheets-Sheet 3 I, Irn/entor-z Thomas C. Lennox;

U Mf M(/.

His Attorney.

Patented July 5, 1927.

UNITED STATES 1,635,002 PATENT OFFICE.

THOMAS C. LENNOX, 0F PITTSFIELD, MASSACHUSETTS, ASSIGNOR TO GENERAL ELEGTRIO COMPANY, A CORPORATION OF NEW YORK.

CONTINUOUS-CURRENT TRANSFORMER.

Application filed August 20, 1925. Serial No. 51,494.

My invention relates to apparatus for transforming direct current energyfrom one voltage to another voltage.

The general scheme to be used is the well known one of converting directcurrent to alternating current'by i'nechanical means such as acommutator or some other device for making and breaking circuits inregular and appropriate sequence, transforming the alternating currentto the desired voltage and rectifying it again to direct current byother suitable contact making devices.

The particular features constituting the subject matter of the presentinvention are means used in the above type of apparatus to cause theload current through the recti fying contacts to have substantially zerovalue at the instant the contacts open so as to avoid arcing orsparking, and means to bring the voltage between the contactssuiliciently close to a zero value when the contacts are about to closeor have just opened, so as to avoid the formation of an arc in advanceof actual contact, or after opening when high voltage is beingcommutated. If these conditions of current and voltage are achievedthroughout the range of load to be carried by the apparatus, the designof successful direct current transformers becomes largely a matter ofthe mechanical design of suitable contact making devices.

The various features of my invention which are believed to be novel andpatentable will be pointed out in the claims appended hereto. For abetter understanding ofmy invention reference is made in the followingdescription to the accompanying drawings in which Fig. 1 is arepresentation of prior art devices which will be referred to indescribing my invention; Fig. 2 is an illustration of myimprovedcommutating arrangements showing the means employed for preventing shortcircuit of'the coils undergoing commutation but which still permits theload current to flow uninterrupted; Figs. 3 and 4 and 5 are illustrations showing various modifications of my invention provided with meansfor supply-- ing the necessary reactive kva. of the transformerapparatus; and Figs. (5 and 7 are curves explanatory of my invention.

In Fig. 1, l0 and 11 represent a trans former arranged in that form ofcircuit known as a ring or polygon. 10 may be considered the primary and11 the secondary. As here represented the apparatus is 16 connected tosupply a low voltage direct.

current circuit 18, bear on the commutator of the secondary. If now thisapparatus is driven in any suitable manner so as to produce relativerotation between the commutators and their brushes, continuous currentwill tend to flow through the primary coils first in one direction andthen in the opposite direction. Thusthe current, although continuous inthe line, is alternating in the I coils 10 and hence it may be expectedto be transformed by the apparatus and reappear in the secondary windingllat corresponding values of voltageand current as determined by theratio of transformation where itmay be rectified by the brushes 16 tosupply the low voltage source.

There are certain inherent reasons why this simple arrangement will notfunction successfully. It is seen that such a device will notsuccessfully receive exciting current for the transformer from thedirect current lines as such current, having to flow in inductivecircuits, would resist interruption by the brushes and cause excessivesparking. It has heretofore been proposed to overcome this objection bydriving'the rotating part of the rectifier by a direct current motorwhich is also arranged to act as a synchronous alternating currentgenerator to supply the needed energizing current'for the transformer,that is, the motor would act as'a synchronous inverter.

Such apparatus as is outlined above forms the basis for many schemes fordirect current transforn'iation and the. inventions claimed hereinconsist largely of improvements and modifications of this type ofapparatus, it being understood that the physical form shown is of noconsequence and may be replaced. by any arrangement which re 'tainssubstantiallythe same general arrangements of circuits and provides formaking and breaking the circuit in the necessary sequence.

Referring to Fig. 1, it will be seen that if complete interruption ofthe direct current is to be avoided it is necessary for the brush tomake contact with the commutator segment it approaches before leavingthe previous one. As a result, the phase of the winding between thesegments must be short cireuited for a short period. If the winding isreceiving alternating current excitation from a synchronous machine,such a short circuit even though occurring during that part of the cyclewhen the voltage reverses and is conse uently small in value, willresult in a considerableflow of current through the brush. If thevoltage per phase is small, this current may be limited to av harmlessvalue by using a brush of high resistance material, but if more than acomparatively few volts are to be used, this method is impracticable.Therefore it appears that the contacts must be arranged so that thepartof the brush, or its equivalent, which touches the one segment mustbe separate from the part which touches the other so that we mayinterpose some device forlimiting the current due to the shortcircuiting of the transformer phase.

Fig. 2 represents such apparatus in which the.positive brushes 19 and 20are connected to the primary of an alternating current transformer T andthe negative brushes 21 and 22 are connected to the primary of atransformer T which will be called for convenience the brushtransformers. It will also be observed that the primary brushes 19 and20 are connected to one winding of the trans former T and the secondarybrushes 23 and.

24 are connected to the other winding of the same transformer. The samearrangement is carried out with respect tothe negative brushes 21, 22,and 25, 26, and transformer T,. The direct current lines 17 and 18 areconnected to middle points of the respective windings of thetransformers T and T Now, if the transformers T and T have the sameratio of turns as the main transformer 10, 11, they will givesubstantially the desired result. That this is so, will be evident if weremember that the load current in the primary is in one directionrepresenting the inflow of power and that in the secondary is in theopposite direction representing the outflow of power as represented bythe arrows at T, whereas the current tending to flow due to theexcitation of the main transformer is the same in each case representinga flow of power from the synchronous machine 27, provided to supply theexcitation current of the main transformer, into the short circuit onthe phase under the brushes, as represented by the arrows at T.Therefore, if the coils are properly opposed in the brush. transformer,the load currents will compensate for each other and only encounter theleakage reactance of the brush transformer while the short circuitcurrents will add to each other in causing induction in the core andthus encounter great reactance and be correspondingly limited in amount.In other words, the short circuit current will flow as exciting currentin the brush transformer and the line current will flow as load current.Thus, if the exciting current and reactance of the brushtransformer aremade small, a minimum of reactance will be inserted in the line circuitsand a maximum in the commutator short circuit. This balance of primarycommutator voltage against secondary commutator voltage is, in so far asI know, original with me and I consider it an important aspect of myinvention.

It will be observed that exciting current for the brush transformersmust through the brushes and'consequently even though it is very smallin amount, it is desirable to bring it to zero value wheuthe contactsopen. Considering those parts of the brush transformer windings betweenbrushes 20 and 21 and the primary lines at the time the brushes 19 and22 are out of contact, the voltage 20 to 21 is obtained from thesynchronous machine 27 and is passing through its maximum value. Thevoltage between the primary lines is constant and obtained from thesource of power 17. The coils of the brush transformer between brush andline are consequently subjected to a varying voltage representing thediiference between these two quantities. Nowif the value of the varyingvoltage from the synchronous machine is greater than the line voltage,current will flow from brush to line, whereas if it is less, currentwill flow from line to brush. If the brush is in the neutral position asregards the excitation from the synchronous generator, the voltage 20 to21 due to the cynchronous machine, starting at a given value when thebrush comes into contact with a segment will rise to a maximum, thenfall to the first value as the brush loses contact. Now, if the linevoltage has an intermediate value, current will first flow into thebrush, then reverse and flow out and then reverse and flow inward.Consequently by properly adjusting the two voltages, the current can beso adjusted as to just come to zero on the last reversal as the contactsopen. This is represented in the curves of Fig. 6 where line Brepresents the direct current line voltage with respect to a zero line0; curve C represents the alternating current excitation voltage betweenbrushes 20, 2l,as it appears rectified by the corresponding brushes;curve D represents the difference between the direct current linevoltagc and the su erimposed alternating current voltage, w ichdifference is what produces the exciting current for the brushtransformer, which exciting current is represented-by the curve E. Thealternating current excitation voltage producing the curve C should beso, proportioned with respect to the direct current line voltage as tobring the exciting current curve E to substantially a zero value whenthe circuit of this phase is opened at the points X. This may beaccomplished by a suitable balance between the direct current supplyvoltage and the excitation voltage supplied by the synchronous generator27, as

for instance by connecting the field coils 28 of the generator 27 acrossthe direct current source, as shown, or 'by any other arrangement suchthat the excitation of' 'generator 27 is proportional to the directcurrent line voltage.

From the above it is evident that the upper and lower halves of thebrush transformers might constitute entirely separate devices connectedfrom line to brush it the proper balance of exciting current is obtainedsuch that it will always be zero as the circuit opens. The sum of. thevoltages from brush 19 to line and from 20 to line where both brushesare in contact must of necessity equal the totalafrom 19 to 2() so thatthe original balance .oifcircuits is not disturbed.

It may not be desirable, however, to use entirely separate brushtransformers for this purpose as when the winding, from 19 to line ismagnetically coupled with that from 20 to line the one will serve tobring the other .to such value of voltage as will prevent any greatvoltage between the brush and the segments itap roaches. This isrepresented in Fig. 7 w ere ((1) indicates the rectified voltage wavesof the two phases, the zero voltage line 0 and thedirect current linevoltage B; curve (11) represents the voltage from brush to the directcurrent line for one phase; curve (0) represents the voltage from brushto the direct current line for the other phase; curve (d) representscurves (1)) and (c) as superimposed as they will be if the brushtransformer windings are placed on the same core and will result in acompromise brush transformer voltage wave substantially as indicated incurve (6). Thus, in Fig. 7 (a), if one brush has a voltage 2 above thedirect current line voltage B when the other lbrush comes into contactwith its commutator segment at a point rep-' resented b the dotted lineX the voltage from the rst mentioned brush to line Wlll cause acorresponding voltage from the other brush to line which will be equaland 0p posite as represented by the oint W to line B. This will lowerthe resu tant line voltage to a value below line voltage andapproximately equal to that due to. the synchronous machine 27 at theinstant of making or breaking contact which is represented by thedistance of point 'Y from line B. As no load current will exist in thebrush not in contact during this eriod, a close magnetic coupling of thecoils will not be necessary to give the desired result. Moreover asthe'total voltage brush to brush 19 to will start as amaximum in onedirection then reverse and finish as an equal former T, it is obviousthat they may be built as one transformer with separate windings on thesame core. 1

From the above, it appears that we can obtain a complete balance of'voltages in such a systenrzvithout sparking. It will be observed thatthe balance between primary and secondary voltage is maintainedregardless of the variation in brush voltage due to the change fromphase to phase. In other words, the total secondary voltage is composedof the sum or difference of the instantaneous values of voltage in mainand brush transformers and bears a constant relation to the primaryvoltage. We can consequently use a sma 1 number of phases withoutintroducing a large ripple in the secondary voltage due to' brushaction.-

Reducing the number of phases to a practical minimum we obtain afour-phase system in which each commutator has but two segments per pairof poles in the inverter or excitation generator). We may also usediametric connection if desired with good economy with so small a numberof phases that all the windings will be active during a large part'ofthe cycle. This is shown in Fig.3. The two main transformer windings notbeing connectedtogether at the star points we may eliminate the brushtransformer T and make T sufliciently large to transform the total brushvoltage. 1

In what follows we will refer to the arrangement shown in Fig. 3 for thesake of simplicity although the principles involved will apply to anyone of many combinations and number of phases.

Serious difficulty will still be encountered if a load current is drawnfrom such an apparatus. If the transformers could be built with zerovalue of leakage roactance the primary load current would flow throughthe apparatus in one direction and the secondary load current'intheopposite direction without reactive effect and consequently would nottend to flash when interrupted by the commutator. As, however, thetransformers must have some leakage reacta'nce, it is obvious that meansmust be provided to reduce the currents to zero at the opening of thecircuits. The action of the commutator converts the direct currents intoalternating current for transformation and the currents in the variouswindings are consequently all alternating or pulsatin The p11 satlngcurrents may be considere as alternating currents imposed on directcurrents. rents, we know that their eifect will be to shift the phaseangle of the secondary voltage away from the primary voltage causing alag in secondary voltage. If we can overcome this tendency and keepthesecondary voltage in phase with the primary voltage, we will be enabledto commutate the currents without flashing. That this is so will berecognized if we remember that the lag in secondary voltage is/theresult of the generation of an alternating voltage in quadrature withthenormal voltage and that this voltage is generated by .the reversal ofthe secondary current. If we oppose this voltage with an equal andopposite one so as to bring the primary and secondary voltages back intophase We will necessarily have provided the voltage required to reversethe current durin commutation. The means I propose for this purpose is asecond syn chronous generator 29 driven by and in exact time phaserelation with theexcitatiom motor inverter or generator 27 which mayobviously be connected to the secondary of the main transformer asillustrated. The windings of this generator will be placed directly, orthrough suitable intermediate transformers, in series with either theprimary or secondary or both coils of the main transformer. Voltage ofthe necessary value and phase position to overcome the reactance voltageof the main and brush transformers and to time bring about the desiredbalance will be generated in this machine 29. This may be done byproviding the generator with fioldcoils 30 suitable for carrying theload current of the direct current lines and passing this currentthrough this field. The generator will thus have an armature in serieswith the transformer windings and a field in series with the directcurrent line.

The voltage of each of the series generator phases must be in quadraturewith the normal voltage of the transformer phase to which it isconnected. If connected in the primary it must lag, if in the secondaryit must lead and if in both it must lag in the primary and lead in thesecondary. The voltage of this generator may then be ad justed bymeans'of an inductive resistance shunt 31 across the field until abalance is obtained between transformer reactance voltage and eneratedvoltage such that commutation is successful. In order that this balancemay be maintained throughout a suitable range of load current, it isnecessary to have the relation between field ampores and armature voltsin the generator similar to the relation between coil amperes andreactance volts in the transformers. Thus if the leakage fluxes in thetransformer occur in material predominatelynonmagnetic the air gap ornon-magnetic por- Considering these alternating cur-' ator, that is, onewhich if mounted on the same shaft will have double the number of poles.This generator may be wound for single phase alternating current and beplaced in series with the brush circuits either directly or throughintermediate transformers I. Such an arrangement is of intermediatetransformer I so that the direct currents are balanced within thetransformer and saturation of the core by unidirectional flux avoided.The direct current is kept out of the series generator coils altogetherby this method. Thevoltage in the series generator 29 should be inquadrature with the normal voltage in the brush transformer T.

Fig. 5 shows a modification employing three rectifying devices and threephase excitation, The brush transformer T in these cases becomes threephase zig-zag connected.

In accordance with the provisions of the patent statutes, l havedescribed the principle of operation of, my invention, ,together withthe apparatus which I new consider to represent the best embodimentthereof; but I desire to have it understood that the apparatus shown anddescribed is only illustrative and that the invention may be carried outby other means.

- What I claim as new and desire to secure by Letters Patent of theUnited States, is:

1. Apparatus for converting direct ourrent of one voltage to directcurrent of a diiferent voltage comprising a main polyphase, transformingapparatus having mechanical rectifying devices associated with itsprimary and secondary windings such that the circuit between thecooperating rectifying contacts are completely opened at the point ofreversal of the alternating current wave, a brush transformer having itsprimary winding connected between the primary rectifying contacts of thesame polarity and its secondary winding connected between the secondaryrectifying contacts of the same clarity, said brush transformer having te same ratio of transformation as the main transformer, whereby thevoltage of commutation of the primary is balanced against the voltage ofcommutation of the secondary. I

2. Apparatus for converting direct current of one voltage to directcurrent of anrepresented in Fig. 4': showing a preferred means ofarranging this circuit by means other voltage comprising a mainpolyphase transforming apparatus arranged to be connected between directcurrent circuits by means of mechanical rectif ing devices respectivelyassociated with tie primary and secondary windings of said maintransformer, an auxiliary brush transformer having its primary windingconnected across the primary rectifying contacts of one polarity, andits secondary connected across the [secondary rectifying contacts of thesame polarity, said brush transformer having the same ratio oftransformation as the main transformer and terminals at neutral pointsof said brush transformer windings for connection to direct currentlines of corresponding polarity.

3. Appar'atus'as claimed in claim'2, characterized by the provision ofan auxiliary synchronous alternator driven in synchronism with therectifiers for'producing the exciting current for said transformers.

4. Apparatus as claimed in claim 2 char- I mechanical rectifierassociated with its primary winding, a mechanical rectifier associatedwith its secondary winding and a brush transformer. having the sameratio as the main transformer inductively connected between the directcurrent primary and secondary rectifying contacts of the same polarityfor balancing the primary voltage of commutation against the sec-.

ondaryyvoltage of commutation.

In witness whereof, I have hereunto set my hand this 11th day of August1925 THOMAS C. LENNOX.

